Colloidal silica refers to a dispersion of silica particles in a liquid phase. In colloidal silica, silanol groups on a surface of silica particles tend to dissociate. This dissociation leads to a high negative charge. The high negative charge leads to inter silica particle siloxane bonding (self-condensation) leading to a gel of the colloidal silica. The colloidal silica is stabilised against inter silica particle siloxane bonding (self-condensation) by the presence of ionic charges on the silica particles, for example, by counter ions of a salt. If a pH of the colloidal silica is in the range of approximately 5-7 or if excess salt is added, then the silica particles in the colloid aggregate and form the gel. By their very nature of being electrostatically stabilised, colloidal silica has a limited shelf-life as it tends to form a gel over time. This is undesirable when the colloidal silica needs to be stored.
Coatings, such as non-stick coatings can be made from a sol-gel process. The sol-gel process can involve hydrolysis and a condensation of silica particles in the colloidal silica in a first mixture with an organoalkoxysilane in a second mixture. The two mixtures can be mixed and applied to a substrate, the resulting mixture is cured to form the coating on the substrate via the sol-gel process. Coatings manufactured this way are often referred to as ceramic coatings and are used for example to coat substrates (such as kitchen utensils) as an alternative to polytetrafluoroethylene (PTFE) coatings.
A paper titled “Coating of methyltriethoxysilane—Modified colloidal silica on polymer substrates for abrasion resistance” by Lee et al. is known from Journal of Sol-Gel Science and Technology 24, pp. 175-180, 2002. The document by Lee et al. discloses that a coating is prepared my providing colloidal silica (Ludox LS, 30 wt %) and mixing this with at least 10% methlytriethoxysilane (MTES, Si(OCH2CH3)3CH3) to form a coating solution. The coating solution is applied to a substrate and cured via the sol-gel process to form the coating on the substrate. The colloidal silica has a limited shelf-life as it tends to form a gel over time. Furthermore the coating solution has a limited shelf-life due to the limited shelf-life of the colloidal silica and a pH of the coating solution. The cured coating cannot be further used in a sol-gel process as the colloidal silica completely reacts with the methlytriethoxysilane to form the coating.
EP 2048116 discloses nano-dispersions. The nano-dispersions comprise nanoparticles, organic solvent, low amounts of water and substantially no dispersants. The nanoparticles can be particles of silica. The nanoparticles can comprise organic surface groups grafted from alkylyalkoxysilanes. The amount of grafting compounds added to the nanoparticles to obtain stable dispersions ranges between 2 and 40 wt %.
WO 2007/104258 discloses a non-stick coating. The non-stick coating is made from two main components of silicon dioxide. One component is colloidal silica comprising pure SiO2 silica particles. The other component is an organoalkoxysilane. The two components are mixed to form a coating solution which is applied to a substrate. The coating solution is hydrolysed and condensed in a sol-gel process to form the non-stick coating on the substrate.
US 2010/0181322 and US 2013/014944 disclose a non-stick coating. The non-stick coating is prepared by making a first mixture of colloidal silica, an alcohol, a pigment and a food grade silicon oil. A second mixture comprises methlytrimethoxysilane (MTMS, Si(OCH3)3CH3) and an acid. The two mixtures are then mixed together and the resulting mixture is applied to a substrate and cured to form the non-stick coating.
In all cases, due to the presence of salts and/or a pH the colloidal silica has a limited shelf-life as it often self-condenses and forms a gel.
In applications where filler and/or pigment particles need to be used this poses further problems. The filler and/or pigments particles contain additional salts in addition to the salts already present in the silica particles and the colloidal silica. In coating applications filler and/or pigment particles form slurries that have a pH which compromises a stability of the silica particles and the resulting colloidal silica. An excess of salt and/or the pH leads to the colloidal silica rapidly forming a gel. Therefore resulting dispersions have a reduced shelf-life. In applications where filler and/or pigment particles are needed, the known technologies are restrictive.
Furthermore when used in applications where organic pigment particles are needed in combinations with the silica particles and the resulting colloidal silica, the organic pigment particles often release significant amounts of salt when dispersed with water. These significant amounts of salt results in an accelerated gelling rate of the colloidal silica and therefore compromises a shelf-life of the product. This is more problematic with coloured organic pigments, which tend to release larger amounts of salt than inorganic pigments. There is therefore a need to provide a colloidal silica which, when used with pigment particles has an improved shelf-life.
Furthermore in applications where the first mixture needs to be mixed with a second mixture, this also means that a final mixture of the two mixtures has a reduced shelf-life in applications such as coatings as mixing of the two mixtures is not uniform due to gel formation.
There is a need to provide silica particles and a colloidal silica with improved stability which overcomes the aforementioned problems.
There is a need to provide silica particles and a colloidal silica with improved stability which can react via the sol-gel process with an organoalkoxysilane which may be used for coating applications.
There is a need to provide a method for the manufacture of silica particles and colloidal silica which is less susceptible to inter silica particle siloxane bonding (self-condensation) with improved shelf-life.
There is a need to provide a kit for coating applications which also overcomes the aforementioned problems. In particular a kit for coating applications which can be used with pigments which is less susceptible to inter silica particle siloxane bonding (self-condensation) with improved shelf-life.